Generation of stable frequency radiation at an optical frequency

ABSTRACT

A system for generating a stable optical frequency from a laser signal having inherent frequency fluctuations. The signal from a injection-controlled pulsed laser is divided into two parts. One part is mixed with the signal from a stable CW laser to generate beat frequencies. These signals are amplified and recombined with the pulsed laser signal in an output modulator. In one embodiment, the difference frequency between the pulsed laser and the reference signal is less than 1000 MHz. The beat frequencies are increased by an X-band mixer to the microwave range where they can be readily amplified in an available broad band amplifier. In another embodiment, the transmitter laser and the reference laser operate at a different frequency in the micorwave range, say, above 5,000 MHz. The beat frequencies are obtained by a high frequency mixer such as a bulk crystal in a waveguide or cavity. In still another embodiment, two independent transmitter lasers generate pulses that occur with a significant time delay. These two signals are combined, amplified and mixed to obtain the desired sideband signals. The subsequent corrective modulation elimintes the frequency fluctuations in each pulse and results in coherent time-delayed pulses.

CROSS REFERENCES TO RELATED APPLICATION

This application is a continuation-in-part of Application Ser. No.06/915/652 filed Oct. 6, 1986 now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an improved system for modulating the outputfrom a laser to remove frequency instabilities.

2. Brief Description of the Related Art

It is known that a laser frequency can be stabilized against thefrequency of an external reference laser by an automatic feedbackcontrol, AFC. This stabilization is achieved by introducing a correctivesignal to the laser frequency tuning mechanism by way of a feedbackloop. The corrective signal is obtained by reference to a laserfrequency signal, for example, from a highly stable low-power CW laser.Various stabilization refinements may be incorporated, includingfrequency stabilization against a narrow Doppler-free resonance asdescribed in U. S. Pat. No. 3,686,585 to Javan and Freed. However, theradiation frequency of lasers fluctuates, under some conditions, at veryhigh speeds requiring a correspondingly high speed correction of thelaser output. The frequency stabilization techniques described abovecannot be used if the frequency fluctuation occurs at a rate higher thanthe response time limit of the laser tuning mechanism. Frequencyvariations occurring from pulse to pulse and within the pulse may bothoccur at rates high enough to preclude correction by the laser tuningtechniques.

In U.S. Pat. No. 4,329,664 the present inventor describes an adaptiveapproach to frequency control, ADFC, for the generation of stablefrequency radiation at an optical frequency. This approach is differentfrom an AFC method in that it does not utilize a feedback loop. A smallsample of the signal from a power laser that produces optical radiationat a frequency, ω, subject to short-term frequency variations iscombined with a signal from a highly stable reference laser to generatea difference beat frequency signal, ω_(m), in the radio frequency rangehaving frequency variations corresponding directly to the fluctuationsof the laser frequency. The resulting signal represents theinstantaneous difference between the primary laser signal and that ofthe reference laser. This difference signal is amplified and recombinedin an modulator with the signal from the primary laser to produce astable optical signal. The system corrects for both intrapulse chirping,frequency fluctuations within the pulse, and for frequency fluctuationsthat occur from pulse to pulse.

In one application, the frequency, ω_(o), of an injection-controlled,pulsed CO₂ laser is maintained at an rf offset with respect to thestable reference CW laser at frequency ω_(r) ·ω_(o) is time dependentbecause of the intrapulse frequency chirp and instabilities. The mixeroutput voltage, V_(b) =V_(o) (t)COS(δt+φ), will appear at the rf beatfrequency δ=ω_(o) (t)-ω_(r). The beat-frequency δ exactly reproduces thetime dependence of ω_(o) (t). A CdTe modulator is driven by thebeat-voltage signal, after amplification in a broad-band, high-gainamplifier. The two rf sidebands o the IR output of the modulator willappear at ω±=ω_(o) (t)±δ(t). Note, for example, for ω_(r) <ω_(o) (t),the down-shifted sideband, ω₋, exactly reproduces ω_(r), if the pulsefrom the primary laser is delayed and adjusted to be the same as thedelay of the amplified rf beat signal in reaching the modulator, thedelays being referenced to the rf mixer input. This time-delaydifference, τ, can readily be adjusted to values below one nsec byadjusting the path-difference in an optical-delay provided at themodulator input. It is desirable to use a broad rf amplifier bandwidthto minimize the rf beat-delay; otherwise a long optical-delaypath-length will be needed to correct for the delay.

In the presence of only a small delay-time difference, τ, the correctedfrequency, ω₋ in this example, will appear slightly shifted: ω₋ =ω_(r) -(dω/dt). For a during-the-pulse chirp rate of, for example, ##EQU1## andτ≦1 nsec, the shift will be ##EQU2## Since τ can be adjusted to besubstantially less than 1 nsec, a sizeably higher frequency chirp ratecan be tolerated, since this 200 Hz limit is readily permissible in mostapplications.

This technique removes the entire intrapulse chirp and frequencyinstabilities to values below 200 Hz in the above example. Inspectionshows that the pulse-to-pulse fluctuations of ω_(o) (t), are alsototally removed from the corrected frequency. For some applications, τas large as tens of nsec can be tolerated, thus considerably relaxingthe bandwidth requirements of the rf amplifier.

The adaptively corrected frequency, ω_(c), will appear as the up-shifted(ω₊) or down-shifted (ω₋) sideband, depending upon whether ω_(r) >ω_(o)or ω_(r) <ω_(o). With respect to an efficient generation of thesidebands, a near-unity single-sideband conversion efficiency canreadily be achieved by driving the modulator at pulsed peak powers inthe multi-kilowatt range.

SUMMARY OF THE INVENTION

The present invention is similar in many respects to the frequencystabilization described above, but the burden for pulsed-modulation at ahigh peak-power for efficient sideband generation, is shifted from therf to the x-band, for which a variety of broad-band, high-gain vacuumtube power amplifiers are available. It is important to provide broadband amplification in order to minimize the time delay correctionrequired at the modulator. However, there the beat signals are moreeasily generated if the difference frequency is in the rf range, say,below 1000 MHz.

In one embodiment, the beat frequencies are generated from two signalshaving a difference frequency below 1000 MHz and these frequencies areincreased by an X-band mixer to the microwave range where they can bereadily amplified in an available broad band amplifier. In anotherembodiment, the transmitter laser and the reference laser operate at adifference frequency in the microwave range, say, above 5,000 MHz. Thebeat frequencies are obtained by a high frequency mixer such as a bulkcrystal in a waveguide or cavity. In still another embodiment, twoindependent transmitter lasers generate pulses that occur with asignificant time delay. These two signals are combined, amplified andmixed to obtain the desired sideband signals. The subsequent correctivemodulation eliminates the frequency fluctuations in each pulse andresults in coherent time-delayed pulses.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 illustrates a system for generating a stable optical frequency inwhich the frequency difference between the transmitter laser and thereference laser is in the rf frequency range and the resulting beatfrequencies are increased to the microwave region in a high frequencymixer prior to amplification;

FIG. 2 illustrates a system in which the transmitter laser and thereference laser operate at frequencies differing by a microwavefrequency and which are combined in a bulk non-linear crystal;

FIG. 3 shows a system similar to that shown in FIG. 2 but in which thefrequency mixing is done in an electro-optic control that produces amicrowave difference is a waveguide, and

FIG. 4 illustrates a system in which pulses from two independenttransmitter lasers are stabilized by a single adaptive control circuit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the embodiment shown in FIG. 1, the signal from aninjection-controlled pulsed laser 2, is passed through an adjustabletime delay 4 and a CdTe modulator 6 to the output. A small sample of thelaser signal, derived by the use of a conventional beam-splitter 8, iscombined, in a mixer 12, with the signal from a stable CW referencelaser 14 that is maintained at an rf offset frequency, for example,below 1000 MHz, from that of the primary laser 2. The rf beat signalfrom the mixer 12 is amplified by a broad-band rf amplifier 16 and thenup-converted to an x-band frequency, say, between 5,200 and 19,900 MHz,in an x-band/rf mixer 18, using an x-band voltage-controlled oscillator(VCO) 22. the mixer output (single-sideband) is then fed to a broad-bandx-band pulsed high-power amplifier 24. The signal level at the x-bandamplifier input need not exceed several watts peak power, thuspermitting the use of existing state-of-the-art broad-band solid staterf amplifiers, x-band VCO's, and mixers at the front end of the x-bandpower amplifier 24. The IR single sideband at the output of themodulator 6 will have the exact adaptive frequency controlcharacteristics discussed above. This embodiment permits the chirp- orpulse-burst, or other waveform, to be introduced by appropriatelyprogramming the VCO 22 to operate in the chirp-burst or the pulse-burstmode. One important difference from earlier systems is the addition ofthe ADFC signal to the front-end driver of the x-band power amplifier24. The long term stability of the pulsed laser is increased by the useof a feedback offset control circuit 26 which is connected between theoutput of the mixer 12 and the frequency control mechanism PZT of theprimary laser 2. The constant offset frequency is maintained by means ofan rf clock 28.

In the embodiment illustrated by FIG. 2, the microwave control signal isgenerated directly from initial beat frequencies obtained at a microwavefrequency. In this case, the stable CW reference laser 14a is chosen tobe one that operates at a microwave offset frequency from the frequencyof the transmitter laser 2a. For example, the primary laser 2a mayoperate with a normal isotopic CO₂ gas mix while the reference laser 14aoperates on an enriched isotopic CO₂ gas mix. It is known in thissituation, the CO₂ transition in the transmitter laser 2a and thereference laser can be chosen to be offset by a microwave frequency.Another alternative is the use of a transmitter laser 2a and a referencelaser 14a in which both gas mixtures are the same isotopic species andthe two lasers operate on the same transition. For the CW referencelaser, however, the output is frequency shifted at a microwave frequencyusing an electro-optic modulator. Because for the reference laser eventens of watts will be sufficient for the embodiment described here, theelectro-optic modulation of the reference laser need not be done at highefficiency. Even an efficiency of 5 or 10% (or less) can be adequate,depending upon the power of the reference laser 14a.

The output of the transmitter laser 2a and the reference laser 14a aremixed in a high-speed detector 12a, which may be a point-contact MOM(metal-metal oxide-metal) or MOS (metal-oxide semiconductor) junction ofthe type previously described by the inventor in U.S. Pat. No.4,329,664. In this case, the beat signal is directly fed to a broad-bandmicrowave amplifier 24a and then to a CdTe crystal modulator 6a as inthe previous embodiment.

FIG. 3 illustates another embodiment in which the transmitter laser 2band the reference laser 14b operate at a microwave offset frequency asin the previous embodiment. The frequency mixing, however, is done in anelectro-optic crystal 32, which may also be a CdTe crystal, thatdirectly produces the microwave difference frequency in a waveguide ormicrowave cavity, diagrammatically illustrated at 34. The resulting beatfrequency is fed through a broad-band x-band microwave amplifier 24b tothe CdTe modulator 6b as before.

The adaptive frequency processor system described here will remove thefrequency fluctuations of injection controlled lasers or other kinds oflasers operating on a single frequency. The embodiments described herealso permit the introduction of the desired waveform envelope of thetransmitted laser pulse.

In the embodiment illustrated by FIG. 4, two pulsed lasers 35 and 36operate at the same CO₂ transition. Each laser is assumed to operate ona single frequency (single mode). The two frequencies are furtherassumed to be e.g., near line center. In this case, the two frequenciesmay differ by e.g., tens of MHz, depending upon their respectiveresonator modes. The two laser outputs will suffer from intrapulse andpulse-to-pulse fluctuations as described previously. Consider the twopulsed laser outputs to be combined with a beam splitter 36 as shown inFIG. 4. The two lasers, however, do not generate pulses at the sameinstant. A trigger circuit 38 is coupled to the power supplies 42 and44, respectively, of the lasers 35 and 36. The output of the combinedbeams consists of two colinear pulses appearing with a time delay. Forexample, each pulse may have a pulse length of 1 usec and the pulse fromthe laser 36 will be delayed by 100 usec with respect to the pulse fromlaser 35. The two pulses will be incoherent with respect to each otherand there will be no relationship between their frequencies.

Consider the combined beams to be subjected to the adaptive frequencyprocessing of any of the earlier embodiments. A stable CW CO₂ referencelaser 46 produces a signal that is combined with a sample from thecombined laser beams, by means of beam splitters 48 and 52. This signalis fed into a CdTe mixer 54 to create the necessary beat notes that arethen amplified by an amplifier 56 and fed into the output modulator 58.the combined beams from the lasers 35 and 36 are passed also through anadjustable time delay 62 to the output modulator 58. The result is thatboth pulses will be processed so that the output of the modulator 58will appear as two delayed pulses at nearly the same frequency. Theadaptive processing in this case introduces coherence between the twodelayed pulses in a reproducible manner. Each time the two lasers 35 and36 are triggered, their fluctuating frequencies ω₁ and ω₂, appear at anarbitrary frequency, e.g., in the CO₂ line center region. Afterprocessing, however, the combined pulses will be at the same frequency,ω_(o), and will be coherent with respect to each other.

The two delayed pulses at the same frequency have important advantagesin laser radar, for example, of the type described in my copendingapplication entitled Bistatic Doppler Laser Radar System forDiscrimination, Tracking and Fire Control filed of even date herewith asSer. No. 915,650. The Fourier transform of the combined pulses atdelayed times have reproducible features with narrow characteristicresonances, the width of which will be determined by the time delaybetween the successive pulses. These narrow features can be used toobtain very small Doppler shifts in a coherent laser radar system.

The adaptive processing described here can be used to remove frequencyfluctuations from either a CW or pulsed laser. It is particularly usefulfor removing the frequency instability of an injection-controlled pulsedCO₂ laser or injection-controlled near UV excimer lasers, such as XeCland others.

As used in this application, the term electro-optic crystal means adevice capable of producing radiation at the difference frequency bymeans of the crystal refractive index non-linearities. CdTe is anexample of such a crystal.

I claim:
 1. In a system for generating a highly stable opticalfrequency, the combination comprisinga pulsed laser for generating laserpulses, said laser pulses being subject to frequency variations, astable CW reference laser, operating at a frequency difference from saidpulsed laser i the microwave band, for producing a frequency stablereference output, a beam-splitter for separating each one of said laserpulses into a first pulse portion and a second pulse portion, ahigh-frequency mixer for combining said first pulse portion with saidfrequency stable reference output to produce a microwave beat frequencysignal, a microwave band broad-band amplifier for amplifying saidmicrowave beat frequency signal, a modulator, the amplified microwavebeat frequency signal being applied thereto, and a path adapted tocouple said second pulse portion to said modulator, wherein saidmodulator produces output laser pulses having said frequency variationsremoved therefrom.
 2. The combination of claim 1 whereinthe differencein frequency between said laser pulses and said frequency stablereference output is greater than 5000 MHz.
 3. The combination as claimedin claim 2 whereinsaid high-frequency mixer is an electro-optic crystal.4. The combination of claim 3 whereinsaid path includes an adjustabletime delay for delaying the second pulse portion of a given laser pulseby an amount that makes said second pulse portion coincident in timewith the amplified beat frequency signal corresponding to said givenlaser pulse at said modulator.
 5. The combination claimed in claim 4whereinsaid pulsed laser is a CO₂ gas laser.
 6. The combination claimedin claim 5 wherein said reference laser is a CO₂ gas laser.
 7. Thecombination claimed in claim 6 whereinsaid pulsed laser is aninjection-controlled laser. PG,20
 8. The combination of claim 1 furthercomprising circuitry for tuning said pulsed laser in response to saidbeat frequency signal to compensate for long term instability of saidpulsed laser.